Desulphurization of ferrous materials using sodium silicate

ABSTRACT

The invention relates to a method for the preparation of a sodium-based reactive desulphurizing agent for use in molten ferrous materials. The sodium in the reactive desulphurizing agent is a sodium silicate, a composition comprising of Na 2 O and SiO 2 . In a second embodiment of the invention, the sodium silicate reactive desulphurizing agent also comprises of an alkali or an alkaline material or other materials, such as oxides of calcium, aluminum and magnesium. Preferred raw materials for the oxides of calcium, aluminum and magnesium are, respectively, lime, alumina and dolomite. The premixed solid reactive desulphurizing agent is brought in contact with the molten ferrous material, allowing the desulphurization or the double replacement of the iron sulphur to take place and produce a ferrous oxide. The sodium in the reactive desulphurizing agent is rendered resistant to combustion or evaporation on contact with the molten ferrous materials by the flux activity of the silica. A metallic solid, such as aluminum, is introduced into the molten ferrous material to complete the reduction of the ferrous oxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

None

FEDERALLY SPONSORED STATEMENT

N/A

REFERENCE LISTING

None

FIELD OF THE INVENTION

The present invention relates to (1) the use of a sodium compound as theprimary desulphurizing reactant in a reactive desulphurizing agent, (2)the option of including other desulphurizing reactants obtained fromother reactive alkali oxides, alkaline oxides or other chemicalcompounds and metallic solids, (3) the method for producing thesodium-based reactive desulphurizing agent, (4) the process of applyingthe sodium-based reactive desulphurizing agent to a molten ferrousmaterial and (5) the process for the deoxidation or reduction of theiron oxide in a molten ferrous material.

BACKGROUND OF THE INVENTION

Ample agreement exists on the growing need for low sulphur steels. Inmost types of steels, the presence of sulphur above 0.015% isundesirable. The increasing stringent requirement for betterdesulphurization of steel is attributed to the need for producing highstrength low alloy steel and steels resistant to hydrogen inducedcracking. In addition, even minute contents of sulphur has perniciouseffects on the manufacturing and finishing of steel products, such asbrittle fractures in welding and fabrication, a tendency to separatealong grain boundaries when stressed or deformed at temperatures nearthe melting point during the hot rolling phase in sheet steelprocessing, and even the influences on as-cast and the processingcharacteristics of the steel in terms of the workability of surfacefinish.

The desulphurizing of molten ferrous materials comprises thoroughlypermeating the molten material with a reactive desulphurizing agentconsisting of (1) metal oxides for replacing the sulphur from the ironand for providing the flux required to float out the spent reactants and(2) the addition of solid metals to complete the deoxidization orreduction the iron. U.S. Pat. No. 4,014,685 teaches the four essentialfactors, for achieving good desulphurization in either a molten iron orsteel, using a reactive desulphurizing agent. The factors are (1) highdesulphurizing agent basicity, (2) high temperature on desulphurizingagent contact with the molten material, (3) low oxygen potential and (4)high desulphurizing agent fluidity.

Over the past several decades, a multitude of methods and processes havebeen set forth for the desulphurization of ferrous molten materials.Most of these methods are based on the reactions of a lime, fluorspar,aluminum oxide, alkali and alkaline metals or alloys, as well as the useof other reactants. An example of these methods is in U.S. Pat. No.3,779,739, which teaches the art of desulphurization using calcium oxideand aluminum oxide or calcium oxide and calcium fluoride, wherein thecalcium oxide increases the basicity of the desulphurizing agent and thealuminum oxide and calcium fluoride increase the fluidity of thedesulphurizing agent. Magnesium oxide has also been advanced in the artof desulphurizing ferrous materials, specifically metallic magnesium inU.S. Pat. No. 6,383,249.

Sodium, like magnesium, is a highly reactive metal, thus ahigh-performing desulphurizer that allows for a very high sulphurreplacement in molten ferrous materials. However, because of its highreactive property, metallic sodium is highly unstable at ambienttemperatures and violently reactive at the temperatures of a moltenferrous material and thus vaporizes explosively on contact with moltenferrous materials, hence metallic sodium, by itself, is ineffectual inremoving sulphur. A preferred composition for the effective use ofsodium as a desulphurizer is a chemical compound, such as in an oxidizedstate in combination with a flux-enhancing compound. A far-reachingattractive feature in advancing the art of using sodium over magnesiumas a desulphurizer is the availability of sodium compounds at asignificantly lower cost.

The art of using sodium oxide, derived from sodium carbonate, as areactant in a desulphurizing agent is described in U.S. Pat. No.4,014,685, U.S. Pat. No. 4,956,010, and U.S. Pat. No. 4,353,739. In noneof these examples, where the art of using sodium oxide as a reactant ina reactive desulphurizing agent is put forth, is the art advancedwherein sodium silicate is used as the source of the sodium oxidedesulphurizing reactant. Nor is a mention made on the preferred form forusing a sodium silicate composition as the sole desulphurizer reactantin a desulphurizing agent.

By itself, the melt point of SiO₂ is about 1580° C. However, once theSiO₂ is mixed or combined with other oxides such as Na₂O, the melt pointof the resulting composition is much lower, around 700° C. The low meltpoint is an important and practical industrial property. On contact withthe molten ferrous material, less thermal energy is required to dissolvethe solid sodium silicate reactive desulphurizing agent; thus, lesscaloric energy is necessary to maintain the ferrous material in itsmolten state as the sulphur replacement progresses. Hence, the use ofsodium silicate as a desulphurizing agent is an efficient and effectivemethod for removing sulphur from ferrous materials.

SUMMARY OF THE INVENTION

A method is disclosed for removing sulphur in molten ferrous materialsusing a sodium-based reactant in a reactive desulphurizing agent. Thepreferred reactant in the reactive desulphurizing agent is sodiumsilicate. A lime, dolomite, an alumina or mixtures thereof, mayoptionally be added to enhance the replacement of the sulphur andcontrol of the fluidity of the spent reactants in the molten ferrousmaterials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, a reactive desulphurizing agent issodium silicate, wherein the chemical composition of the sodium silicatecomprises of y parts of SiO₂ to x parts of Na₂O and wherein, yetfurther, the ratio, y/x, is anywhere from 0.5 to 5. The SiO₂ and theNa₂O are fused at high temperatures. In the preferred embodiment of theinvention the fusion takes place in a glass tank at temperatures from900 to 1500° C. Other types fusion units, such as a rotary furnace, canalso be used for the present invention provided the end results areconsistent with the intent and scope of the invention.

The sodium silicate reactive desulphurizing agent is brought intointimate contact with sulphur-laced molten ferrous materials. In thepreferred embodiment of the invention, the molten ferrous material isdrawn, in predetermined amounts, and laid in a layer to cover thesurface of a desulphurizing vessel. The layer of molten ferrous materialis then covered by a predetermined amount of sodium silicate reactivedesulphurizing agent. The process is repeated at least once until allthe molten ferrous material available is desulphurized within the volumecapacity of the vessel. As the desulphurization of the molten ferrousmaterials progresses, the spent reactants fluidize the surface and areremoved from the desulphurizing vessel. To complete the reduction ordeoxidation of the iron, a metallic solid is introduced into the moltenferrous material. In the preferred embodiment of the invention themetallic solid is an aluminum rod.

In another embodiment of the invention, the reactants of the reactivedesulphurizing agent comprise: sodium silicate, as prepared in thepreferred embodiment, and oxides of calcium, magnesium and aluminum. Thepreferred materials for said oxides are a lime compound such aslimestone, dolomite and an alumina composite, respectively.

According to the invention, the preferred composition of the reactantsin reactive desulphurizing agent, without the carbon dioxide comprises:Na₂O from about 7 to about 50% SiO₂ from about 7 to about 50% CaO is≦45% MgO is  ≦8% Al₂O₃ is ≦25%

These and other advantages and features of the present invention will bemore fully understood with the examples thereof

EXAMPLE 1

A reactive desulphurization agent, based on sodium, was prepared from acomposition of a sodium silicate and oxides of aluminum, magnesium andcalcium. The parts ratio of the silicon oxide to the sodium oxide was2.3:1, resulting in a sodium silicate composition with about 29.7%, byweight, of sodium oxide and about 71.3%, by weight, of silicon oxide. Inaddition to the sodium silicate composition, alumina, dolomite and limewhere added in the amounts given in Table 1. The components wherecombined using a blade mixer in three approximately equal batches toobtain consistent mixtures. The weight percentages of the oxides in theunspent reactive desulphurizing agent are listed in Table 2. Aftermixing, the components were placed super-sacks and shipped to the siteof the Basic Oxygen Furnace (BOF). TABLE 1 Desulphurizing AgentComponent Weight Component Kg Sodium silicate 234.5 Alumina 284.1Dolomite 218.8 Lime 535.4 Total 1272.8

TABLE 2 Desulphurizing Agent Composition (pbw) Percent by WeightReactive Oxides CaO  53.33% Na₂O  7.19% MgO  5.70% Flux Enhancers Al₂O₃ 18.35% SiO₂  15.42% Total 100.00%

At the BOF site, initially, 70 metric tons of molten steel were drawninto a tap vessel and covered by 400 kg of the sodium-based reactivedesulphurizing agent, as described in Table 1. An additional 70 metrictons of molten steel were again drawn into the tap vessel and againcovered the sodium-based reactive desulphurizing agent. The controlsparameters were maintained constant to provide oxygen at 356 ppm. Asolid metallic aluminum wire was injected into the molten steel todeoxidize the iron oxide, with the oxygen reduced to 2.5 ppm. During thedesulphurization of the molten steel, neither the presence of toxicvapors nor the presence of sodium where detected in the immediatesurrounding area. The spent reactants of the reactive desulphurizingagent remained dissolved and fluid during the desulphurization process.Measurements performed on samples taken prior and after desulphurizationindicate that the sulphur content of the molten steel was reduced from0.012% to about 0.0088%.

EXAMPLE 2

In the second example, the sodium-based reactive desulphurizing agentbased again was prepared from a sodium silicate and oxides of aluminum,magnesium and calcium. The parts ratio of the silicon oxide to thesodium oxide was maintained as before, 2.3:1, resulting in a sodiumsilicate composition with about 29.7%, by weight, of sodium oxide andabout 71.3%, by weight, of silicon oxide. However the amounts the sodiumsilicate composition, alumina, dolomite and lime added were slightlychange, as given by weight amounts in Table 3. As before, the componentswhere combined using a blade mixer in successive batches. The batchweights for this example were not reported. The weight percentages ofthe oxide in the final sodium-based reactive desulphurizing agentcomposition are listed in Table 4. After mixing, the components wereplaced super-sacks and shipped to the BOF site. TABLE 3 DesulphurizingAgent Component Weight Component Kg Sodium silicate 225.8 Alumina 364.8Dolomite 200.9 Lime 408.5 Total 1200.0

TABLE 4 Desulphurizing Agent Composition (pbw) Percent by WeightReactive Oxide CaO  45.50% Na₂O  7.00% MgO  7.00% Flux Enhancers Al₂O₃ 25.00% SiO₂  15.50% Total 100.00%

Essentially the desulphurization procedure at the BOF site, as outlinedin Example 1, was repeated in this example, with the exception that theoxygen levels were lower both during the desulphurization phase and thedeoxidation phase. Initially, 70 metric tons of molten steel were drawninto a tap vessel and covered by 400 kg of the sodium reactivedesulphurizing agent, as described in Table 1. An additional 70 metrictons of molten steel were again drawn into the tap vessel and againcovered with a layer of the sodium reactive desulphurizing agent. Thecontrols parameters were maintained constant to provide oxygen at 226ppm. Metal aluminum wire was injected into the molten steel to deoxidizethe iron oxide. In this example, the oxygen was reduced to 1.7 ppm.During the process, neither the presence of toxic vapors nor thepresence of sodium where detected in the immediate surrounding area. Thespent reactants of the reactive desulphurizing agent, as before,remained dissolved and fluid during the desulphurization process.Measurements performed on samples taken prior and after desulphurizationindicate that the sulphur content of the molten steel was reduced from0.0153% to about 0.0135%.

A further variation of the process consists of a number of sequentialcharges of lesser amounts of the molten steel charge and provide saidmolten charge with a layer of the reactive desulphurizing agent, whereineach layer of the reactive desulphurizing is in intimate contact withthe molten material.

It will be appreciated that the instant specifications and claims areset forth by way of illustrating and not limiting the present invention,and that various modifications and changes may be made without departingfrom the spirit, scope and intent of the present invention.

1. A method for desulphurizing a molten ferrous material, comprises areactive desulphurizing agent, said reactive desulphurizing agentfurther comprises of desulphurizing reactants, wherein at least onedesulphurizing reactant contains a sodium compound.
 2. A method asdefined in claim 1, wherein the sodium compound is in a sodium silicatecomposition.
 3. A method as defined in claim 2, wherein the sodiumsilicate composition comprises: a. a sodium oxide, Na₂O; and a siliconoxide, SiO₂; wherein further, b. x parts of Na₂O and of y parts of SiO₂;wherein yet further, c. the ratio, y/x, is anywhere from 0.5 to
 5. 4. Aprocess of claim 3, wherein the fusion of the sodium oxide and thesilicon oxide takes place in a glass tank.
 5. A process of claim 3,wherein the fusion of the sodium oxide and the silicon oxide takes placein a rotary furnace.
 6. A method as defined in claim 1, wherein thereactive desulphurizing agent comprises of a sodium silicate and atleast one other non-sodium desulphurizing reactant.
 7. A method asdefined in claim 6, wherein the non-sodium desulphurizing reactants, inthe reactive desulphurizing agent, are selected from a group of alkaliearth metal compounds, alkaline metal compounds and other metals,compounds, composition and combinations thereof
 8. “Deleted”.
 9. Amethod as defined in claim 7, wherein the desulphurizing agent comprisesof a sodium silicate and non-sodium desulphurizing reactants, whereinthe sources for said non-sodium desulphurizing reactants are obtainedfrom calcined materials: a lime, dolomite and an alumina.
 10. A methodas defined in claim 9, wherein lime, dolomite and alumina provide: a.calcium oxide; b. magnesium oxide; and c. aluminum oxide.
 11. A methodas defined in claim 9, wherein the desulphurizing agent comprises asolid mixture of from about 7 to about 50% by weight of sodium oxide,from 7 to about 50% by weight of silicon oxide, less than or equal toabout 45% by weight of calcium oxide, less than or equal to about 8% byweight of magnesium oxide, and less than or equal to about 25% by weightof aluminum oxide.
 12. A method according to claim 9, wherein thereactive desulphurizing agent is placed in intimate contact with amolten ferrous material for the purpose of replacing the sulphurcontaminant in the iron.
 13. “Deleted”.
 14. “Deleted”.
 15. “Deleted”.16. A method according to claim 12, wherein at least on metallic solidis introduced into the desulphurized molten ferrous material todeoxidize or reduce the iron in the molten ferrous material.
 17. Amethod as defined in claim 1, wherein the reactive desulphurizing agentis placed in intimate contact with molten ferrous materials.
 18. Amethod according to claim 17, wherein at least one metallic solid isintroduced into the desulphurized molten ferrous material to deoxidizeor reduce the iron in the molten ferrous material.
 19. A method asdefined in claim 1, wherein the ferrous material is selected from agroup comprising: iron, pig iron, iron alloy, steels, mixtures thereofand other ferrous materials and wherein said ferrous material iscontaminated with sulphur.
 20. A method derived from claim 1, whereinthe desulphurizing agent is also a fluxing agent.
 21. A method asdefined in claim 20, wherein the fluxing agent enhances the process ofreplacing of sulphur in the ferrous material.